This invention relates to a process for producing krypton and xenon in which liquid oxygen containing krypton and xenon in small concentrations accumulating in the main condenser evaporator of a conventional air separation plant is rectified to concentrate krypton and xenon, and particularly relates to a process producing krypton and xenon in which heat necessary for the rectification is provided by argon cycle whereby the whole system can be made compact and operation thereof can be easily performed.
In producing krypton and xenon, it is a normal procedure to concentrate liquid oxygen accumulating in the main condenser evaporator of the air separation plant by rectification to obtain liquid oxygen containing krypton and xenon in higher concentrations and to further rectify thus concentrated liquid to produce pure krypton and xenon gases. Referring to FIG. 1, in the above prior process the liquid oxygen extracted from the main condenser evaporator 1 is fed through line 2 to a first concentrating column 3 where it is rectified and then most of oxygen in the liquid is discharged from the top of the first concentrating column 3 through line 4 and concentrated liquid accumulates in a condensation section 5 at the bottom of the column 3. This rectification results in concentration of krypton and xenon, and also results in concentration of hydrocarbons contained in the liquid oxygen such as methane. Particularly, the enrichment of methane is liable to occur explosion. To avoid this explosion hazard, the concentrated liquid is vaporized by a heater 7 after flowing out through line 6 and then the vaporized hydrocarbons are burned in a catalytic combustion cylinder or reactor 8. Thereafter, the obtained vapor containing the combustion products are introduced through line 9 into one of a switchover-type adsorber 10 where water and carbon dioxide are removed by adsorption, the purified vapor is led through line 11 to a heat exchanger 12 where it is cooled, and is fed to a second concentrating column 14 by line 13, where the fed gas mixture is rectified. Oxygen gas is extracted by line 16 from the top of a condensation section 15 disposed at the top of the second concentrating column 14, and after cooling the gas mixture in the heat exchanger 12 it is withdrawn by line 17. At the bottom of the second concentrating column 14, there accumulates more concentrated liquid mixture of krypton and xenon, which is extracted by line 18 and introduced into conventional purifying and separating steps, in which krypton and xenon are separately recovered.
In the above process, gases separated by the air separation plant are usually used for imparting heat necessary for the rectification to recover krypton and xenon. For example, nitrogen gas is extracted as a heating source of the concentrating column 3 from the lower column of the air separation plant. The nitrogen gas is introduced by line 19 into the concentrating column 3 for reboiling where it is liquefied, and then the liquefied nitrogen is extracted and returned by line 20 to the air separation plant. To the second concentrating column 14 oxygen gas which has been separated by the air separation plant and then pressurized is fed through line 21, and generates upflowing gases therein. Furthermore, liquid oxygen obtained also from the air separation plant is usually supplied as a cooling source to a condensation section 15 of the second concentrating column 14 where it generates reflux liquid which is needed for rectification, and thereby liquid oxygen is vaporized. The vaporized gas is returned to the air separation plant by line 23. These heating and cooling units necessitate a system of a large number of long pipes connecting the krypton- and xenon-recovering plant to the air separation plant, and hence makes assemblage of the system greatly laborious. Furthermore, these units can afford disturbances to the air separation plant, and thereby causes the operation of the air separation plant to be unstable. Therefore, the application of the krypton- and xenon-recovering plant to air separation plants already built is not easily achieved according to the above prior heating process. In addition, the nitrogen gas as a heating source for rectification must be used at a relatively high pressure, i.e., about 5 atg. This requirement produces another disadvantage in pressure proof of the plant.